There has been used a tape cartridge called "IBM 3480" tape cartridge having a single reel having a magnetic tape wound thereon as disclosed in Japanese Patent Application Laying-Open No. 171,773/1983 for a magnetic instrument such as a magnetic storage for backing up informations as a computer peripheral instrument. Such a tape cartridge is provided with a locking member disposed at a central axis of the reel to lock the reel so as to prevent the reel from rotating. In a prior art, the locking member is forced and displaced by reel drive means when the tape cartridge is set on the reel drive means so as to be relieved from its locking.
However, since the locking member is stationary in the reel while the reel drive means is rotatable, the locking member and the drive means are slidably engaged with each other, which disadvantageously causes vibrations and noises to occur.